The present application, as presently envisioned, relates to systems and methods for the manufacture of continuous, three-zone reinforced, geometrically symmetrical, microporous membranes having three distinct pore zones, each zone being formed from at least one of a plurality of different pore size producing dopes, more particularly to systems and methods for the continuous manufacture of continuous, reinforced, three-zone microporous membrane including a scrim having two sides at least substantially encapsulated within any one of a plurality pore size producing first dopes produced from a single mother dope batch and at least one additional dope presently preferably produced from the same single mother dope batch coated onto each side of the substantially encapsulated scrim prior to the first dope being quenched and, most particularly, to systems and methods for the manufacture of a geometrically symmetric, continuous, reinforced membrane having three distinct pore zones including a scrim at least substantially and preferably, completely encapsulated by a relatively large pore size middle zone produced from any one of a plurality of different pore size dopes, which may be continuously produced from a single mother dope batch and two outer zones, one on each side of the middle zone, at least one of the three zones having a pore size at least about twenty (20%) percent greater than at least one of the other zones.
Microporous phase inversion membranes are well known in the art. Microporous phase inversion membranes are porous solids which contain microporous interconnecting passages that extend from one surface to the other. These passages provide tortuous tunnels or paths through which the liquid which is being filtered must pass. The particles contained in the liquid passing through a microporous phase inversion membrane become trapped on or in the membrane structure effecting filtration. A slight pressure, generally in the range of about one half (0.5) to about fifty (50) psig (pounds per square inch gauge) is used to force fluid through the microporous phase inversion membrane. The particles in the liquid that are larger than the pores are either prevented from entering the membrane or are trapped within the membrane pores and some particles that are smaller than the pores are also trapped or absorbed into the membrane pore structure within the pore tortuous path. The liquid and some particles smaller than the pores of the membrane pass through. Thus, a microporous phase inversion membrane prevents particles of a certain size or larger from passing through it, while at the same time permitting liquid and some particles smaller than that certain size to pass through. Microporous phase inversion membranes have the ability to retain particles in the size range of from about 0.01 or smaller to about 10.0 microns or larger.
Many important micron and submicron size particles can be separated using microporous membranes. For example, red blood cells are about eight (8) microns in diameter, platelets are about two (2) microns in diameter and bacteria and yeast are about 0.5 microns or smaller in diameter. It is possible to remove bacteria from water by passing the water through a microporous membrane having a pore size smaller than the bacteria. Similarly, a microporous membrane can remove invisible suspended particles from water used in the manufacture of integrated circuits in the electronics industry. Microporous membranes are characterized by bubble point tests, which involve measuring the pressure to force either the first air bubble out of a fully wetted phase inversion membrane (the initial Bubble Point, or xe2x80x9cIBPxe2x80x9d), and the higher pressure which forces air out of the majority of pores all over the phase inversion membrane (foam-all-over-point or xe2x80x9cFAOPxe2x80x9d). The procedures for conducting initial bubble point and FAOP tests are discussed in U.S. Pat. No. 4,645,602 issued Feb. 24, 1987, the disclosure of which is herein incorporated by reference. The procedure for the initial bubble point test and the more common Mean Flow Pore tests are explained in detail, for example, in ASTM F316-70 and ANS/ASTM F316-70 (Reapproved 1976) which are incorporated herein by reference. The bubble point values for microporous phase inversion membranes are generally in the range of about two (2) to about one hundred (100) psig, depending on the pore size and the wetting fluid.
Methods and Systems for preparing the dope used to produce microporous membrane are known in the art. There are numerous methods of preparing the dope. Prior methods of dope preparation are discussed in background section of U.S. patent application Ser. No. 09/022,295 mentioned above under related applications.
It is also known that processing relatively large bodies of dope, such as that used in the production of microporous phase inversion membranes, is accompanied by many difficulties such as the need to formulate separate dope batches for each size pore phase inversion membrane produced as well as the problems in controlling the temperature of the dope during the batching process.
As was pointed out in the ""295 application, during production runs of microporous phase inversion membrane, it is important to produce microporous phase inversion membrane having the desired pore size and/or pore size distribution.
As summarized in the ""295 application, in the prior batch formulation process, the dope formulation (solvent, nonsolvent, polymer ratio) was key to controlling pore size in the microporous phase inversion membrane. Using the batch formulation method as a predictive control of pore size in microporous phase inversion membrane, microporous phase inversion membrane having a specific pore size was produced from a specifically formulated dope batch.
As described in the ""295 application, thermal manipulation to change the pore size in a membrane produced from a dope has long been recognized and has been used in reprocessing out of specification dope, as discussed therein. However, this recognized property of the dope was dependent on raising the temperature of the dope to a temperature higher than that to which the dope had previously been processed. While one prior patent mentioned in the ""295 application discussed controlling the process temperature as one factor in enabling continuous production of material with fixed or variable pore size from a single batch of nylon 46 solution, that prior patent failed to provide any specific temperatures other than a wide temperature range. Further, in the only example relative to varying pore size, the patent combined process temperature manipulation with the composition of the dope and the composition of the bath to effectuate the pore size change but only in one direction, from smaller to larger. There was no apparent effort to control the temperature of the solution at a specific temperature or any effort to try to lower the temperature of the solution to produce a smaller pore size.
Following the teachings of that particular prior patent, using thermal manipulation to change the pore size and viscosity of the mixture, as the solution is heated to higher temperatures, the viscosity of the dope becomes such that it might not be usable in a solution casting operation, unless controlled. Specifically, as the particular solution is heated to higher temperatures, processing problems will most likely be encountered including those related to viscosity, degassing of volatile components, foam formation and quenching problems, without adequate viscosity control.
The methods taught in that prior patent are not applicable to Marinaccio style Nylon 66 dopes and the membrane products produced therefrom, for the following reasons: 1) the patent is directed toward attempting to produce a skinned membrane, with a radically altered pore structure just below the qualifying skin layer. In this method, the quality and integrity of the skinned membrane is completely dependent on the quality of the first few microns of surface thickness. With this method, even the smallest imperfection (air entrapment, substrate fiber breach, etc.) in the skin will destroy the integrity of the product. For this reason, the methods disclosed in the patent must restrict the casting solution viscosity to a very narrow practical range, to ensure wetting of the substrate, minimization of entrapped air, and xe2x80x9csmooth, even coating of the mixture,xe2x80x9d to ensure the integrity of the finished membrane product. There is, however, a practical limit to the solution viscosity; therefore a single stage thermal treatment and hot casting would potentially lower the viscosity to an impractical point, thus limiting the useful range of resultant pore sizes. 2) Additionally, the single stage thermal treatment and hot casting would be harmful to the resulting product, in that the volatile non-solvent components of the Marinaccio style dope (Methanol and Methyl Formate) will de-gas in an uncontrolled manner upon casting at a temperature above thirty-four degrees (34xc2x0) C. (boiling point of Methyl Formate), and form bubbles, voids and other imperfections in the surface and matrix of the membrane. These voids are not desirable in commercial microporous membrane.
In the end, the teaching of that prior patent appears ambiguous as to the effect of temperature alone on pore size because smaller pore size materials could result primarily from 1) different casting dope solution formulations, or 2) higher proportions of solvents in the bath as it was known that a range of different pore sizes could be produced from a single solution by changing the proportions of solvents in the bath.
As summarized in the background of the ""295 patent application, the prior art can be described as a non-real time predictive batch-type process that uses formulation to initially control pore size and bulk reheating as a predictive thermal manipulation to produce a predictive pore size to correct an improperly formulated batch, or improperly controlled initial mix cycle, sheer speed control to introduce the nonsolvent in the preparation of the dope as a batch of liquid to be processed into a membrane and bath solvent control in order to vary the pore size. In some prior art, discussed above, at the end of the formulation process, the dope had a viscosity related to the process temperature. There was no apparent attempt to independently control the viscosity of the dope prior to moving the dope to a membrane production apparatus.
As with dope preparation, methods and systems for producing reinforced microporous membrane are also known in the art. A number of prior patents were discussed in the ""979 and the ""816 patent applications that have been incorporated by reference. While there appears to have been considerable effort to (1) develop methods and processes for preparing dopes which when processed into microporous membrane produce microporous membrane having a specific pore size and (2) methods and apparatus for manufacturing reinforced microporous membrane, none of these efforts appear to have resulted in a system and method including the preparation of a mother dope connected to a reinforced microporous membrane manufacturing apparatus that is capable of producing reinforced three-zone microporous membrane having any one of a plurality of different dopes in any one of the three zones.
Thus, there is a need for systems and methods for continuously manufacturing a relatively thin, geometrically symmetrical, continuous, monolithic, reinforced, polymeric microfiltration membrane having at least three independent and distinct pore size performance zones (one reinforced performance zone, presently preferably, central to the membrane structure, and two outer non-reinforced performance zones including at least one outer qualifying performance zone on one side of the central reinforced zone and a second outer non-qualifying prefilter performance zone on the other side of the central performance zone or, two outer qualifying performance zones, one on each side of the central zone) progressing through the thickness of the membrane, each zone being continuously joined throughout the membrane structure utilizing at least one mother dope batch to provide any one of a plurality of different pore size dope to any one of the three zones. Such systems and methods should produce a three-zone membrane structure by a highly robust, single unit operation, with on-line pore size and layer thickness attribute control. Such systems and methods should produce a three-zone membrane that meets the industry""s long recognized need for superior performance and greater flexibility of triple layer composite structures having any one of a plurality of pore sizes in any one of the three zones. Such systems and methods for producing a three-zone membrane should provide for the relatively inexpensive manufacture in a continuous process with the capability of changing the pore size in any of the zones including changing dope batches. Such systems and methods of manufacturing a three-zone membrane should eliminate the complex production of traditional laminated single layer structure membrane and increase the range of pore sizes and manageable handling thickness that are provided by the non-reinforced zones. Such systems and methods for manufacturing a three-zone membrane should have a geometrically symmetrical structure having improved utility, flexibility, and processability into finished industrial forms (pleated cartridges, etc.) while assuring structural integrity in any one of plurality of different pore sizes in each of the three zones.
Such systems and methods of manufacturing a three-zone membrane should provide a membrane having a minimum functional thickness and maximum throughput at minimal pressure drops, high integrity and be economically produced such that there is any one of plurality of different pore sizes in each of the three zones. Such systems and methods for manufacturing a three-zone microporous membrane should include the formulation of at least one mother dope batch at a temperature equal to or below the target temperature for producing the smallest desired pore size of the possible plurality of pore sizes for each zone to be produced from the at least one mother dope batch. Such systems and methods for manufacturing a three-zone microporous membrane should provide for the elevation of selected portions of the at least one mother dope batch to any one of a plurality of target temperatures such that microporous membrane having any one of a plurality of corresponding pore sizes can be simultaneously produced from at least one mother dope batch. Such systems and methods for the manufacture of a three-zone microporous membrane should provide for the temperature control of at least a portion of the at least one mother dope batch to about xc2x10.2xc2x0 C. of a target temperature prior to that portion of the dope prepared at the target temperature and after cooling being transferred to at least one dope application apparatus of a reinforced, three-zone microporous membrane manufacturing system at a processing site. Such systems and methods for the manufacture of the three-zone microporous membrane should provide for the accurate control of the temperature seen by substantially all of that portion of the dope to about xc2x10.15xc2x0 C. prior to that portion of the dope being transferred to at least one dope application apparatus of the reinforced, three-zone microporous membrane manufacturing system. Such systems and methods for manufacturing three-zone microporous membranes should eliminate the necessity for preparing at least one dope batch according to individual unique formulations for each pore size, thus resulting in significant cost savings and flexibility in the usage of dope batches. Such systems and methods for manufacturing reinforced, three-zone microporous membranes should also provide the ability to selectively change the pore size of at least one zone of the three-zone microporous membrane being produced from the at least one mother batch after a certain amount of at least one zone of the reinforced, three-zone microporous membrane has been produced at one specific pore size and begin producing reinforced, three-zone microporous membrane having another pore size in that same zone utilizing the same at least one mother dope batch.
An object of the present application is to provide systems and methods for manufacturing three-zone, reinforced, continuous, non-laminated, geometrically symmetrical microporous membrane possessing structural integrity.
Another object of the present application is to provide systems and methods for manufacturing reinforced, three-zone continuous, non-laminated symmetrical microporous membrane exhibiting low pressure drop and high flow rate across the membrane.
A further object of the present application is to provide systems and methods for manufacturing reinforced, three-zone continuous, non-laminated, geometrically symmetrical microporous membrane which is particularly suitable for the filtration of biological or parenteral fluids.
Yet a further object of the present application is to provide systems and methods for manufacturing reinforced, three-zone continuous, non-laminated, geometrically symmetrical microporous membrane which is particularly suitable for the filtration of high purity water for the electronics industry.
Yet another object of the present application is to provide systems and methods for manufacturing such a three-zone, continuous, reinforced, non-laminated, geometrically symmetrical microporous membrane.
In accordance with these and further objects, one aspect of the present application includes a system for manufacturing three-zone microporous membrane, the system comprising: at least one vessel for containing a ternary phase inversion polymer mother dope, the dope having been exposed to a mixing temperature which is sufficient to effect dissolution and equilibrium mixing of the polymer, solvent and nonsolvent, the dope contained therein being maintained at a temperature sufficient to stabilize and maintain the mixture after completion of the mixing; at least one pressure means, operatively connected to the at least one vessel, for moving the dope from the at least one vessel to a dope processing site; a dope transportation system, operatively connected to the at least one vessel and the dope processing site, for transfer of dope from the vessel to the dope processing site; at least one thermal manipulation means, operatively connected to the at least one vessel and the dope processing site, for transforming the dope into any one of a plurality of different possible pore size producing dopes; and at least one dope application means operative at the dope processing site and operatively connected to the at least one thermal manipulation means, for applying the dope at the dope processing site such that three-zone microporous membrane is produced.
Another aspect of the present application includes a system for manufacturing three-zone microporous membrane, the system comprising: at least one vessel for containing a ternary phase inversion polymer mother dope, the dope having been exposed to a mixing temperature sufficient to effect dissolution and equilibrium mixing of the polymer, solvent and nonsolvent, the vessel and the dope contained therein being maintained at a temperature sufficient to stabilize and maintain the mixture after cooling from the mixing temperature; a dope transportation system, operatively connected to the at least one vessel and to the dope processing site, for transporting the dope from the vessel to the dope processing site; at least one pressure means, operatively connected to the at least one vessel, for moving the dope from the at least one vessel to a dope processing site; at least three thermal manipulation means, operatively connected to the at least one vessel, the dope transportation system and the dope processing site, for transforming the dope from the at least one vessel to the three thermal manipulation means into any one of a plurality of different possible pore size producing dopes; and at least three dope application means, operatively connected to each of the three thermal manipulation means for application of the dope delivered to the dope processing site such that three-zone microporous membrane is produced.
Still another aspect of the present application includes a three-zone microporous membrane prepared by a process comprising the steps of: providing at least one vessel for containing a ternary phase inversion polymer mother dope; formulating a ternary phase inversion polymer mother dope in the vessel to effect dissolution and equilibrium mixing of the polymer, solvent and nonsolvent; maintaining the mother dope in the vessel at a temperature sufficient to stabilize and maintain the dope formulated after cooling from the formulation temperature; providing a dope processing site having at least one dope application means; operatively connecting the at least one vessel to the dope processing site such that the mother dope is transported from the at least one vessel to the dope processing site; operatively positioning at least one thermal manipulation means between the at least one vessel and the dope processing site; thermally manipulating the mother dope transported from the at least one vessel to the at least one thermal manipulation means into any one of a plurality of different dope possible pore size producing dopes; and applying a predetermined one of the plurality of different possible pore size producing dopes received from the at least one thermal manipulation means to a scrim at the dope processing site to produce reinforced, three-zone microporous membrane.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.